Advantages of Hydrogen Why Hydrogen? Think individually about what you know about hydrogen and its advantages, discuss with your neighbor(s), and be prepared to share your answer. 3/15/2018

Disadvantages of Hydrogen Why not hydrogen? Think individually about what you know about hydrogen and its disadvantages, discuss with your neighbor(s), and be prepared to share your answer. 3/15/2018

Hydrogen Production • There is no natural source of hydrogen • Hydrogen can be considered as a energy carrier, not an energy source. • To supply the hydrogen for energy needs, economical processes are needed to produce hydrogen from abundant energy sources 3/15/2018

Hydrogen Production - Coal • Advantages – Can be implemented using current technology – U. S. has enough coal to make all of the hydrogen the economy needs for >200 years 1 – Lost cost for hydrogen • Disadvantages – – Produces more CO 2 than other technologies (carbon sequestration? ) Same environmental concerns as electricity generation from coal Centralized production Purification and separation of hydrogen at high temperatures is challenging 3/15/2018 1 “The Hydrogen Economy”, The National Academies Press, Washington, D. C.

Hydrogen Production – Solar Energy • Solar energy can be harnessed to produce hydrogen in several ways: – Photovoltaic cells: solar energy is converted to electricity which drives water electrolysis – Photoelectrochemical methods – Thermochemical methods • Use heat from a solar collector to drive a cycle which converts water to hydrogen 3/15/2018

Hydrogen Production – Solar Energy • Recent work has studied the combination of these two processes in a single nanoscale process. Photon absorption creates a local electron-hole pair that electrochemically splits a neighboring water molecule • This requires a material that is both stable in aqueous environments and has a small bandgap so that solar energy can be absorbed. • Possible solutions: – Dye-sensitized photocells that accumulate energy from multiple low-energy photons to inject higher-energy photons into semiconductor – Doped oxide semiconductors with reduced bandgaps 3/15/2018

Hydrogen Storage • Storing hydrogen in a high energy-density form is a key part of the hydrogen economy • Liquefaction of hydrogen is prohibitively expensive (~30% of energy content is lost in liquefaction). Compression to 10, 000 psi costs ~11% of hydrogen’s energy content. • Hydrogen storage media are required that store a lot of hydrogen in a small volume and can easily desorb hydrogen on demand 3/15/2018

Summary and Conclusions • Hydrogen is extremely attractive because of its environmental implications, and because of hydrogen in fuel cells is efficient • Many options are being considered for hydrogen production. Production from renewable sources is the most attractive long-term, but has the most technical barriers at the current time • Hydrogen storage is a critical issue that needs to be overcome for implementation of hydrogen in transportation applications 3/15/2018